Pr5 (BO4 )3-x (BO3 )x (F,OH)2.67 O0.28 (x≈1.6), a boron-containing fluoride-oxoapatite-like compound, was obtained by the application of high-pressure/high-temperature synthesis. It exhibits a superstructure of the apatite type with a tripled c lattice parameter (space group P63 /m) and shows complex anion disorder along the 63 screw axis and occupation of distorted octahedra, as well as almost trigonal planar sites, by oxygen and fluorine atoms. Furthermore, a distinct BO4 /(BO3 +F) group disorder is found; 46 % of the sites being occupied by BO4 groups and 54 % by BO3 groups, with a fluoride ion located near the missing oxygen atom. The rare earth cations in the 4f sites exhibit a specific distorted tricapped trigonal prismatic coordination with a mean metaprism twist angle of 21.3°. The crystal structure of Pr5 (BO4 )3-x (BO3 )x (F,OH)2.67 O0.28 (x≈1.6) shows much "flexibility" resulting in split and off-site positions of all other rare earth cations. The title compound therefore combines many structural features of apatite-like compounds, for example biologically highly-important carbonated apatites, shedding more light onto the complex structural chemistry of apatites.
Keywords: apatite; borates; high-pressure chemistry; lanthanides; solid-state structures.
© 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.